Apparatus to Control Device Temperature Utilizing Multiple Thermal Paths

ABSTRACT

Apparatus to control device temperature includes a thermal fluid path which transfers a thermal transfer fluid so that the device is heated or cooled in response to the thermal transfer fluid; a temperature sensor in communication with a controller and being operable to sense a temperature in response to the thermal fluid path; a first path in thermal contact with a heating assembly, which first path is fluidly connected at a first end to the thermal fluid path; a second path in thermal contact with a cooling assembly, which second path is fluidly connected at a first end to the thermal head fluid path; a third path fluidly connected at a first end to the thermal fluid path; a supply apparatus which supplies thermal transfer fluid to a confluence of a second end of the first path, a second end of the second path, and a second end of the third path; and a valve assembly being operable, in response to the controller, to regulate flow of thermal transfer fluid flow through the first, the second, and the third paths.

This is a continuation of a patent application entitled “Method andApparatus for Controlling Temperature” having Ser. No. 13/411,573 whichwas filed on Mar. 4, 2012, which patent application is a continuation ofa patent application entitled “Method and Apparatus for ControllingTemperature” having Ser. No. 11/725,091 which was filed on Mar. 16, 2007and which issued as U.S. Pat. No. 8,151,872 on Apr. 10, 2012, theentireties of all such identified prior patent applications and patentsare hereby incorporated herein in their entirety.

TECHNICAL FIELD OF THE INVENTION

One or more embodiments of the present invention relate to method andapparatus for controlling temperature, and more specifically to methodand apparatus for controlling temperature of an electronic device duringtesting.

BACKGROUND OF THE INVENTION

Electronic devices such as semiconductor integrated circuits (“ICs”) aretypically tested under conditions that include, particularly for complexdevices, several temperatures (for example, including temperatureextremes). In addition, functional testing is performed on relativelyexpensive test equipment, and time taken to carry out such testing isimportant for economic reasons. Thus, time taken to establish testtemperatures ought to be minimized to minimize testing time and costentailed in using such test equipment.

In common testing practice, devices under test (“DUTs”) are brought to atest temperature in a thermal soak step in a test sequence. In morerecent testing practice, tests are carried out at several temperatures,such temperatures often including a low temperature that is well belowroom temperature. The use of such multiple test temperatures, andcondensation of water at low temperatures, renders a thermal soak stepimpractical for helping to establish such multiple test temperatures. Inaddition, increasingly, DUTs are cycled through a set of testtemperatures while they are mounted on a test head, which cycling takesup valuable test equipment time.

Recently, in response to the above, considerable effort has beenexpended to find methods to establish and cycle DUT temperature rapidlyduring a test sequence while the DUT is mounted on test equipment(referred to herein as rapid thermal conditioning of a DUT).

One commonly used method to provide rapid thermal conditioning of a DUTentails placing a fluid-cooled test head in contact with a DUT, wherethe temperature of the test head is modulated by resistive heating, forexample, utilizing a resistive heating element attached directly to thetest head. In accordance with this method, to raise the temperature ofthe test head, additional current is passed through the resistiveheating element. Likewise, to lower the temperature of the test head,current passed through the test head is reduced. This method isinefficient in that the test head is both heated and cooled at the sametime. Further, compromises in design that enable resistive heating tochange the temperature of the test head also act to reduce the thermalefficiency of the test head. In particular, the temperature of theresistively heated test head is typically controlled by: (a) sensing thetemperature of the DUT, and (b) using this temperature to controlcurrent supplied to the test head. In further such embodiments, powersupplied to the DUT is also sensed, and the power supplied to the DUT isused to: (a) anticipate temperature changes of the DUT, and (b)accommodate such anticipated temperature changes by adjusting currentsupplied to the resistive heating element attached directly to the testhead.

Another method to provide rapid thermal conditioning of a DUT entailsfirst cooling, and subsequently heating, a stream of air that isdirected onto the DUT, where heating of the stream of air is done byresistive heating thereof. Because of this, heating is relatively rapid.In addition, the method of first cooling and then heating a flow of airis inefficient, and limits the efficacy of the method. In furtheraddition, the large amount of energy expended in both heating andcooling the air (or other thermal transfer fluid) limits the amount ofair that can be processed in a practical system.

Yet another method to provide rapid thermal conditioning of a DUTentails mixing hot and cold thermal transfer fluids to provide a thermaltransfer fluid at a predetermined temperature to a heat exchangerthermally connected to the DUT. In accordance with this method, byadjusting the ratio of flows of the thermal transfer fluids, thetemperature of the DUT may be controlled within a band of temperaturesbetween a temperature of the hot thermal transfer fluid and atemperature of the cold thermal transfer fluid. However, because a hotthermal transfer fluid must be mixed with a cold thermal transfer fluidto obtain a thermal transfer fluid having an intermediate temperature,this mixing process is inefficient.

In light of the above, there is a need in the art for method andapparatus that solves one or more of the above-identified problems.

SUMMARY OF THE INVENTION

One or more embodiments of the present invention satisfy one or more ofthe above-identified needs. In particular, one embodiment of the presentinvention is an apparatus to control device temperature that comprises:a thermal fluid path which transfers a thermal transfer fluid so thatthe device is heated or cooled in response to the thermal transferfluid; a temperature sensor in communication with a controller and beingoperable to sense a temperature in response to the thermal fluid path; afirst path in thermal contact with a heating assembly, which first pathis fluidly connected at a first end to the thermal fluid path; a secondpath in thermal contact with a cooling assembly, which second path isfluidly connected at a first end to the thermal head fluid path; a thirdpath fluidly connected at a first end to the thermal fluid path; asupply apparatus which supplies thermal transfer fluid to a confluenceof a second end of the first path, a second end of the second path, anda second end of the third path; and a valve assembly being operable, inresponse to the controller, to regulate flow of thermal transfer fluidflow through the first, the second, and the third paths.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of an apparatus useful to controltemperature of a device under test (“DUT”), which apparatus isfabricated in accordance with one or more embodiments of the presentinvention;

FIGS. 2A, 2B, and 2C are schematic representations of a fluid mixingvalve shown in FIG. 1 at times t=t_(a), t=t_(b), and t=t_(c),respectively;

FIG. 2D shows graphs of temperatures TT3(t) and TT4(t) and flows Q₀(t)and Q₁(t) as a function of time; and

FIG. 3 is a schematic representation of an apparatus useful to controltemperature of a device under test (“DUT”), which apparatus isfabricated in accordance with one or more further embodiments of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of apparatus 1000 that isfabricated in accordance with one or more embodiments of the presentinvention, which apparatus 1000 is useful to control temperature (forexample and without limitation, set and maintain temperature) of deviceunder test 1010 (“DUT 1010”) (such as, for example and withoutlimitation, a microelectronic device and more specifically, asemiconductor integrated circuit). To perform testing using apparatus1000: (a) DUT 1010 is mounted in a fixture, for example and withoutlimitation, a socket, that provides signal contact, for example andwithout limitation, electrical contact, between DUT 1010 and automatictest equipment (“ATE”), for example and without limitation, electroniccircuitry that provides test signals to, and receives responses from,DUT 1010; and (b) DUT 1010 is maintained in good thermal contact withthermal head 1020 that controls the temperature of DUT 1010.

In accordance with one or more embodiments of the present invention, thetemperature of DUT 1010 is established (for example, is set andcontrolled) by changing the temperature of thermal head 1020, and thetemperature of thermal head 1020 is established (for example, set andcontrolled), in turn, by flowing a thermal transfer fluid throughthermal head 1020 (where the term “fluid” will be understood toencompass both liquids and gases). In accordance with one or moreembodiments of the present invention, thermal head 1020 comprises: (a) aplate having a surface area configured to couple to DUT 1010 to transferheat to and from DUT 1010 by way of heat conduction; and (b) a heatexchanger connected to the plate to set a temperature of the surfacearea of the plate by heat conduction. In accordance with one or moresuch embodiments, the plate spreads heat to present a uniformtemperature to DUT 1010. In addition, and in accordance with one or moresuch embodiments, the plate has low thermal capacity (i.e., the plate isincapable of storing much thermal energy) and high thermal conductivity(i.e., the plate is capable of transferring thermal energy rapidly).Rapid rates of change of temperature are enabled, in part, by minimizingthe heat capacity of the plate as much as practicable. However, a needto reduce the heat capacity of the plate should be balanced against aneed for high thermal conductivity so that heat may be more rapidlytransferred between the plate and the heat exchanger. This balancingenables the plate to achieve rapid thermal equilibrium with the heatexchanger when the temperature of the heat exchanger is changed. Inaccordance with one or more embodiments of the present invention, abalance between low heat capacity and high thermal conductivity isstruck by manufacturing the plate from a thin sheet (for example andwithout limitation, 0.060 inches thick) of a highly thermally conductivematerial such as, for example and without limitation, copper. Inaccordance with one or more embodiments of the present invention,thermal head 1020 is a part of a test head (not shown for sake of easeof understanding one or more embodiments of the present invention).

As shown in FIG. 1, and in accordance with one or more such embodiments,to ensure good thermal contact between DUT 1010 and the plate, optionalconductive coatings and structures may be placed on the plate to improvethermal conductance to DUT 1010 (improving thermal conductance improvestemperature set and control performance). For example, a contactingmaterial may be attached to the plate to contact DUT 1010—for exampleand without limitation, a compliant sheet of metal impregnated plasticcan be attached to the DUT 1010 side of the plate. However, thecontacting material need not be as thermally conductive as the materialof the plate. In further addition, optionally, fixture (socket)assemblies used to receive a DUT may allow helium gas to be injected(this allows helium to displace air between thermal head 1020 and DUT1010) since helium is more thermally conductive than air, and thereby,to improve performance. In accordance with one or more embodiments ofthe present invention, the plate is substantially planar since, inpractice, most DUTs have a flat or planar lid or case that serves as amating surface. Of course, the size and shape of the contact surface ofthe thermal head may be configured to mate with a size and shape of theparticular DUT. Alternatively, a suitably configured mating element,formed from a thermal conductor, can be placed between thermal head 1020and DUT 1010 (a mating element may be desirable to accommodate specificphysical characteristics of the DUT or to concentrate heat transfer incertain areas of the DUT). As such, it should be understood that theterm plate is used to refer to any device or portion of a device thatuniformly spreads heat. Thus, in accordance with one or more suchembodiments of the present invention, thermal interface material 1030(for example and without limitation, a material such as eGRAF HiThermavailable from GrafTech International Ltd. of Cleveland, Ohio) may bedisposed between, and in thermal contact with, thermal head 1020 of thetest head and DUT 1010 to increase thermal coupling between thermal head1020 and DUT 1010.

As indicated in FIG. 1, thermal head 1020 has conduits or fluidchannel(s) through which thermal transfer fluid flows. In accordancewith one or more such embodiments, such channels may be microchannels,for example and without limitation, for use preferably when the fluid isa gas. For example and without limitation, in accordance with one ormore such embodiments, thermal head 1020 comprises a block of thermallyconductive material with microchannels for fluid flow within the block.In addition, and in accordance with one or more further embodiments ofthe present invention, thermal head 1020 includes a set of channels thatare interdigitated so that walls between the channels are relativelythin to maximize thermal conductivity, reduce thermal gradients, andpromote uniform temperatures across a surface of a plate that overlaysthe interdigitated channels. In accordance with one or more such furtherembodiments of the present invention, the set of channels includes asingle serpentine channel that switches back and forth. In accordancewith one or more still further embodiments of the present invention, theplate may be a lid for covering the interdigitated channels—using theplate to cover the channels enables the plate to contact the thermaltransfer fluid to promote heat transfer between the thermal transferfluid, as well as between DUT 1010 and thermal head 1020.

In accordance with one or more embodiments of the present invention,thermal head 1020 may be integrated into an ATE so that thermal head1020 is in good thermal contact with DUT 1010 during functional testing.In addition, and in accordance with one or more embodiments of thepresent invention, depending on whether the ATE is for use in testingpackaged devices or bare dies (unpackaged bare chips), an electricaltest path for electrically testing DUT 1010 may be either a test headfor testing packaged semiconductor devices or a prober for testing diesof a semiconductor wafer, respectively. Still further, in accordancewith one or more embodiments of the present invention, the ATE maycomprise a device handler that includes a chuck that receives DUT 1010.In such a case, depending on whether the ATE is for testing packageddevices or bare dies, the chuck may be capable of receiving and holdingeither a packaged semiconductor device or a semiconductor wafer,respectively.

In accordance with one or more embodiments of the present invention,temperature TT4 of thermal head 1020 may be sensed using any one of anumber of methods that are well known to those of ordinary skill in theart including use of thermocouple sensors, thermistors, resistivesensors, diode sensors, IR emission sensors, and any other means forsensing temperature. For example and without limitation, thermal head1020 may include a suitably placed channel configured to carry athermocouple sensor wire that communicates with, and enables, controller1100 to monitor temperature TT4 of thermal head 1020.

As further shown in FIG. 1, temperature TT4 of thermal head 1020 ismeasured by sensor 1040 comprising, for example, and without limitation,a type K thermocouple junction, in thermal contact therewith—as furtherindicated by FIG. 1, temperature TT4 is transferred to controller 1100(sensor 1040 and controller 1100 are in communication) where controller1100 comprises, for example, and without limitation, an Omron CPM2Cmicroprogrammable controller having a CPM2C-TS001 thermocouple sensorinput module (available from Omron Electronics, LLC of Schaumburg,Ill.). As one of ordinary skill in the art can readily appreciate, thistransfer may occur by having sensor 1040 transmit measurements tocontroller 1100 or by having controller 1100 poll sensor 1040 inaccordance with any one of a number of methods that are well known tothose of ordinary skill in the art. In accordance with one or more suchembodiments, temperature TT4 of thermal head 1020 is regulated bythermal transfer fluid flowing in circuit 1050 through thermal head1020. In accordance with one or more such embodiments, the thermaltransfer fluid flowing through thermal head 1020 is circulated in aclosed loop system that conserves heat stored in the thermal transferfluid, thereby increasing the energy efficiency of apparatus 1000.

In accordance with one or more embodiments of the present invention, thethermal transfer fluid ought to: (a) have a low and relatively flatviscosity over the desired temperature range so that it can be pumped;(b) have a thermal capacity which is high enough over the desiredtemperature range so that it can serve as an efficient heat exchangemedium; (c) optionally, be a safe chemical so that no injuries willresult if any part of the human body is exposed thereto; and (d)optionally, be a dielectric so that it will not electrically short anycircuit onto which it might be spilled. For example and withoutlimitation, the thermal transfer fluid may include, water, glycol-watermixtures, water-salt mixtures, FLUORINERT™ a fluorocarbon based fluid(available from 3M Corporation of St. Paul, Minn.), GALDEN® fluid(available from Solvay Chemicals, Inc. of Houston, Tex.), silicone oils,hydrocarbon oils, compressed air, CO2, helium, nitrogen,helium-hydrogen, and other gas mixtures or thermal transfer fluids. Inaccordance with one or more such embodiments of the present invention,the hydrostatic pressure of the thermal transfer fluid as it flowsthrough thermal head 1020 is set to provide a flow rate which results ingood thermal efficiency of heat transfer to and from thermal head 1020(the flow rate of a thermal transfer fluid may be adjusted since thetemperature of a surface area of a plate to which DUT 1010 is contactedhas a functional relationship to the flow rate and temperature of thethermal transfer fluid). As such, this is largely a function of thethermal head design used in a particular application. In addition, flowrates may be varied across a temperature range, with a higher flow ratebeing used with higher thermal transfer fluid temperatures and a lowerflow rate used for lower temperatures due to a typically higherviscosity. For example and without limitation, for an embodiment usinghelium as the thermal transfer fluid, a hydrostatic pressure of between0 psig and 600 psig, and preferably between 80 psig and 200 psig, may beused (where psig is pressure is pounds per square inch gage—aboveatmospheric pressure). In accordance with one or more such embodimentsof the present invention, for example and without limitation, thethermal transfer fluid is dry nitrogen at a pressure of about 150 psigas measured, for example and without limitation, at exit port 510 ofpump 500 that circulates the thermal transfer fluid through circuit1050. In accordance with one or more such embodiments, a hermeticallysealed scroll compressor available from Copeland Corporation of Sidney,Ohio may be used to circulate the high pressure nitrogen gas (as is wellknown in the art, a scroll compressor is a type of gas compressor usedin refrigeration units). In accordance with one or more suchembodiments, the high pressure nitrogen circulates at a rate of aboutthree (3) cubic feet per minute. Further, in accordance with one or moreembodiments of the present invention, the thermal transfer fluid may bepressurized to prevent cavitation in liquids or to increase the heatcapacity of gas phase thermal transfer fluids. Still further, inaccordance with one or more embodiments of the present invention, theflow rate of the thermal transfer fluid may be varied by pump 500 inresponse to signals sent from controller 1100.

As shown in FIG. 1, and in accordance with one or more embodiments ofthe present invention, high pressure nitrogen output from exit port 510of compressor 500 passes through fan-assisted, convective cooler 520(i.e., fan-driven cooler of thermal transfer fluid in tubing) to removeexcess heat caused by compression of the nitrogen. However, it should beunderstood that further embodiments exist wherein such cooling ofnitrogen output from exit port 510 may not be necessary, for example andwithout limitation, in embodiments having low flow designs wherecompressor heating is not significant.

As further shown in FIG. 1, upon exiting convective cooler 520, the highpressure nitrogen passes through secondary side 540 of counter-flow heatexchanger 530, and then to fluid mixing valve 1060 (also referred toherein as proportioning valve 1060). In accordance with one or moreembodiments of the present invention, heat is transferred betweenthermal transfer fluid flowing in fluid channel(s) in secondary side 540and thermal transfer fluid flowing in fluid channel(s) in primary side550 (sometimes referred to as the input side of a counter-flow heatexchanger) of counter-flow heat exchanger 530, thereby bringing thermaltransfer fluid flowing out of secondary side 540 (sometimes referred toas the output side of a counter-flow heat exchanger) to a temperaturenear that of thermal transfer fluid flowing into primary side 550. Inaccordance with one or more embodiments of the present invention, acounter-flow heat exchanger may be designed so that the temperature ofthermal transfer fluid flowing out from secondary side 540 will bewithin several degrees centigrade of the temperature of thermal transferfluid flowing into primary side 550, notwithstanding a temperaturechange of as much as 100° C. in thermal transfer fluid as it flowsthrough secondary side 540 (such as units available from FlatPlate, Inc.of York, Pa.). In accordance with one or more such embodiments, becauseof such a thermal efficiency, counter-flow heat exchanger 530 is able totransfer thermal energy from thermal transfer fluid entering primaryside 550 to thermal transfer fluid entering secondary side 540, therebyconserving thermal energy in thermal transfer fluid flowing in circuit1050. Advantageously, in accordance with one or more such embodiments ofthe present invention, conservation of thermal energy in thermaltransfer fluid circulating in circuit 1050 applies to a case where thetemperature of the thermal transfer fluid is higher than the ambienttemperature as well as to a case where the temperature of the thermaltransfer fluid is lower than the ambient temperature. By way of example,assume that: (a) nitrogen at 150 psig enters primary side 550 ofcounter-flow heat exchanger 530 at 100° C. and exits secondary side 540at a temperature of about 97° C.; (b) a scroll compressor embodiment ofpump 500 operates in an ambient temperature of 27° C.; and (c) nitrogenoutput from the compressor is cooled by convective cooler 520 to atemperature of about 30° C. before it enters secondary side 540 ofcounter-flow heat exchanger 530. As is illustrated by this example, pumpassembly 525 comprising counter-flow heat exchanger 530, scrollcompressor 500, and fan-assisted, convective cooler 520 is able tocirculate thermal transfer fluid with a small loss of thermal energyfrom the thermal transfer fluid, notwithstanding operation of compressor500 at ambient temperature and compressive heating of the thermaltransfer fluid.

As further shown in FIG. 1, and in accordance with one or moreembodiments of the present invention, thermal transfer fluid flows frompump assembly 525 to proportioning valve 1060 that distributes thethermal transfer fluid between straight-through path 1070, hot path1080, and cold path 1090. In FIG. 1, proportioning valve 1060 is shownin a neutral position in which thermal transfer fluid is distributedentirely to straight-through path 1070. In this configuration, thethermal transfer fluid flows directly through thermal head 1020. Thus,in the configuration of apparatus 1000 shown in FIG. 1, thermal transferfluid flows continuously in circuit 1050 through thermal head 1020, pumpassembly 525, proportioning valve 1060, straight-through path 1070, andthence back to thermal head 1020. In an ideal quiescent state in which:(a) counter-flow heat exchanger 530 is 100% efficient; (b) DUT 1010dissipates no energy; and (c) the various components of apparatus 1000are thermally insulated, then, the temperature of the circulatingthermal transfer fluid remains constant in time. As a result, no heatwould need be added to, or taken from, the thermal transfer fluid tomaintain temperature in this ideal quiescent state. However, inaccordance with one or more embodiments of the present invention, aswill be described below, deviations from the ideal quiescent state areaccommodated by adding heat to the circulating thermal transfer fluid byrouting a portion of the thermal transfer fluid through hot path 1080,or by removing heat from the circulating thermal transfer fluid byrouting a portion of the thermal transfer fluid through cold path 1090.

In accordance with one or more embodiments of the present invention, ifpower dissipated by DUT 1010 is increased, the temperature of DUT 1010is maintained constant by mixing a controlled amount of thermal transferfluid from cold path 1090 into the stream of thermal transfer fluidflowing in straight-through path 1070. In accordance with one or moresuch embodiments, a small increase of temperature TT4 of thermal head1020 due to power dissipation in DUT 1010 is detected by sensor 1040 andnoted by controller 1100. In response to this detected increase,controller 1100 causes a signal (for example, pulses) to be sent tostepper motor 1110, which signal causes stepper motor 1110 to rotatelead screw 1120 and, thereby, to move valve slider 1130 attached theretoin a downward direction. As a result, valve slider 1130 partially opensexit port 1220 of proportioning valve 1060 leading to cold path 1090 bya controlled amount. It should be noted that stepper motor 1110, leadscrew 1120, and proportioning valve 1060 may also be referred to hereinas a valve assembly that operates in response to controller 1100, andstepper motor 1110, lead screw 1120, and valve slider 1130 may also bereferred to herein as a valve slider assembly that operates in responseto controller 1100. As one of ordinary skill in the art can readilyappreciate, the controlled amount can be determined routinely andwithout undue experimentation depending on the properties of the thermaltransfer fluid such as heat capacity and flow rate. As a result, aportion of the circulating thermal transfer fluid flows through coldpath 1090 to mix into the stream of thermal transfer fluid fromstraight-through path 1070, thereby reducing temperature TT3 sensed bysensor 1200 (as further indicated by FIG. 1, temperature TT3 istransferred to controller 1100, for example and without limitation, inthe manner described above regarding temperature TT4). This reduction ofthe temperature of the circulating thermal transfer fluid causes areduction in the temperature of thermal head 1020, thereby acting tomaintain DUT 1010 at a nearly constant temperature. The principles ofusing feedback from a detected temperature to set and maintain atemperature are well known in the art, a commonly understood examplebeing thermostatic control of room temperature by feedback control of afurnace. In addition, and in accordance with one or more embodiments ofthe present invention, electrical circuits, using analog components,digital components, or a combination may be used to implement controller1100 and to carry out its functionality described herein. For example,and without limitation, controller 1100 may include a microprocessorthat is programmed using any one of a number of methods that are wellknown to those of ordinary skill in the art. In accordance with one ormore such embodiments, software implementations can be written in anysuitable language, including without limitation high-level programminglanguages such as C+, mid-level and low-level languages, assemblylanguages, application-specific or device-specific languages, ladderlanguages, and graphical languages. In further addition, such softwarecan run on a general purpose computer such as a Pentium, an applicationspecific pieces of hardware, or other suitable devices. In addition tousing discrete hardware components in a logic circuit, the logic mayalso be performed by an application specific integrated circuit (“ASIC”)or other device. In further addition, various embodiments will alsoinclude hardware components which are well known to those of ordinaryskill in the art such as, for example and without limitation,connectors, cables, and the like. Moreover, at least part of thisfunctionality may be embodied in computer readable media (also referredto as computer program products) such as, for example and withoutlimitation, magnetic, magnetic-optical, and optical media, used inprogramming an information-processing apparatus to perform in accordancewith one or more embodiments of the present invention. Thisfunctionality also may be embodied in computer readable media, orcomputer program products, such as a transmitted waveform to be used intransmitting the information or functionality.

In another deviation from an ideal quiescent state, apparatus 1000 maybe used to raise the temperature of DUT 1010 by a controlled amount. Thesequence of events involved in raising the temperature of DUT 1010 isbest understood by reference to FIGS. 2A-2C which show positions ofproportioning valve 1060 at times ta, tb, and tc, respectively. In aquiescent state at ta, as shown in FIG. 2A, valve slider 1130 is in amid-position in which thermal transfer fluid is channeled intostraight-through path 1070. At time t0, controller 1100 causes valveslider 1130 of proportioning valve 1060 to be moved upward by activatingstepper motor 1110 to turn lead screw 1120 holding valve slider 1130.Aperture 1230 in valve slider 1130 uncovers top exhaust port 1140 ofvalve slider 1130, thereby allowing a regulated amount of thermaltransfer fluid to flow into hot path 1080 where it is heated beforerejoining the flow of thermal transfer fluid from straight-through path1070 at junction 1240 (refer to FIG. 1).

A configuration of proportioning valve 1060 after opening a flow to hotpath 1080 is shown in FIG. 2B at time t=tb. In addition, FIG. 2D showsgraphs of temperatures TT3(t) and TT4(t), and of flows Q0(t) and Q1(t)as a function of time (Q0(t) is flow in straight-through path 1070 andQ1(t) is flow in hot path 1080). As one can readily appreciate from FIG.2D, flow Q1(t) of thermal transfer fluid in hot path 1080 increases withtime after controller 1100 causes proportioning valve 1060 to move att=t0. Correspondingly, temperature TT3(t) of the combined flow ofthermal transfer fluid, as measured at sensor 1200 (refer to FIG. 1),increases with time after t=t0. Further, as thermal head 1020 is heatedby the flow of thermal transfer fluid therethrough, temperature TT4(t)of thermal head 1020 increases toward temperature TT3(t) of the thermaltransfer fluid. Flow Q1(t) of thermal transfer fluid in hot path 1080further increases with time as controller 1100 causes valve slider 1130to be moved to open top exhaust port 1140 to hot path 1080, therebyincreasing temperature TT3(t) of the combined flow above a predeterminedset point temperature TS. This “overshoot” of temperature TT3(t) of thethermal transfer fluid above set point temperature TS accelerates therise in temperature TT4(t) of thermal head 1020, thereby enabling thetemperature TT4(t) to reach its set point temperature more rapidly. Theuse of “overshoot” of a forcing function is well known in the art ofthermal control systems, where “overshoot” is commonly used to reducethe time needed to reach a set temperature. As temperature TT4(t) ofthermal head 1020 approaches set point temperature TS, controller 1100causes valve slider 1130 to be adjusted to moderate flow Q1(t) ofthermal transfer fluid through hot path 1080, while increasing flowQ0(t) through straight-through path 1070. At time t=tc, the temperatureof thermal head 1020 is within an acceptable predetermined toleranceband of set point temperature TS. Then, controller 1100 causes valveslider 1130 to be moved to allow a small flow of thermal transfer fluidthrough hot path 1080, as shown in FIG. 2C, to: (a) maintain temperatureTT4(t)≈TS; and (b) offset heat loss in counter-flow heat exchanger 530and heat loss due to imperfect insulation along circulation path 1050.Note that in the example discussed above and illustrated in FIGS. 2A-2D,for ease of understanding one or more embodiments of the presentinvention, flow Q2(t) in cooling path 1090 is set to zero. However, inpractice, and in accordance with one or more embodiments of the presentinvention, a small amount of thermal transfer fluid flow Q2(t) may beallowed in order to maintain operation of refrigeration unit 700 and toprevent floodback of refrigerant into compressor 710.

In accordance with one or more embodiments of the present invention, thetemperatures of hot path 1080 and cold path 1090, respectively, may beregulated to assist in controlling temperature TT4 of thermal head 1020more effectively. In accordance with one or more such embodiments, andas shown in FIG. 1, temperature TT2 of cold path 1090 (i.e., TT2 is thetemperature of heat exchanger 1400 as measured by sensor 1410 in thermalcontact therewith—as further indicated by FIG. 1, temperature TT2 istransferred to controller 1100, for example and without limitation, inthe manner described above regarding temperature TT4) is established bya refrigeration unit 700 comprising: (a) compressor 710; (b) air-cooled,condenser 720 with associated fan 730; (c) expansion valve 740; and (d)evaporation tube 750 that is thermally connected to cold path 1090 byheat exchanger 1400 (as indicated in FIG. 1, heat exchanger 1400 hasfluid channel(s) for thermal transfer fluid flowing therethrough in coldpath 1090 and for refrigerant flowing through refrigeration unit 700).In accordance with one such embodiment of the present invention,compressor 710 and condenser 720 with fan 730 are contained in a ⅓ HPCOPELAND® refrigeration unit charged with type R404A refrigerant that isavailable from Emerson Climate Technologies, Inc. of St. Louis, Mo.; andexpansion valve 740 is a Sporlan bipolar, stepper-controlled valveavailable from Sporlan Division—Parker Hannifin Corporation ofCleveland, Ohio (www.sporlan.com) which is connected to driveelectronics in controller 1100. In accordance with one or more suchembodiments, by way of example, an increase in power dissipation in DUT1010 is offset by an increase in flow of thermal transfer fluid throughcold path 1090, thereby slightly warming elements in cold path 1090. Tocompensate for such warming, controller 1100 sends a signal that causesstepper-controlled, expansion valve 740 to be actuated to releaseadditional refrigerant into expansion tube 750, thereby providingadditional cooling to offset warming caused by the increase in powerdissipation in DUT 1010. One of ordinary skill in the art willappreciate that embodiments of the present invention are not limited tothe described method and components to establish a cold path, and thatfurther embodiments of the present invention exist wherein a cold pathmay be fabricated which utilize, for example and without limitation,thermoelectric coolers, thermoelectric coolers linked to a cold plate,two-stage refrigeration units, vortex coolers, and so forth.

In accordance with one or more embodiments of the present invention, andas shown in FIG. 1, temperature TT1 of hot path 1080 (i.e., TT1 is thetemperature of heat exchanger 1450 as measured by sensor 1460 in thermalcontact therewith—as further indicated by FIG. 1, temperature TT1 istransferred to controller 1100, for example and without limitation, inthe manner described above regarding temperature TT4) is regulated bydissipation of electrical power in heat exchanger 1450 through whichthermal transfer fluid flows in hot path 1080. In accordance with one ormore such embodiments, electrical current is passed through resistivecartridge heater 1470 (for example, one or more resistive cartridgeheaters available from Omega Corporation of Stamford Conn.) that isembedded in heat exchanger 1450 which has fluid channel(s) for thermaltransfer fluid in hot path 1080. In accordance with one or moreembodiments of the present invention, controller 1100 causes current tobe supplied to heater 1470 in response to detection of condition(s) thatrequire additional heat in hot path 1080. In accordance with one or moresuch embodiments, temperature TT1 of hot path heat exchanger 1450detected by sensor 1460 is used by controller 1100 to control operationof apparatus 1000 described above.

In accordance with one or more embodiments of the present invention,proportioning valve 1060 shown in FIG. 1 comprises manifold 1600 havingone input port 1610 on a first side, and three output ports 1140, 1630,and 1220 on a second side. As was described above, valve slider 1130 ismoved by lead screw 1120 which is rotated by stepper motor 1110 to openand close access between input port 1610 and output ports 1140, 1630,and 1220 by incrementally varying amounts of motion. In accordance oneor more embodiments, both output port 1140 to hot path 1080 and outputport 1220 to cold path 1090 are not opened at the same time. Althoughnot shown, O-ring seals may be used around each of input port 1610 andoutput ports 1140, 1630, and 1220 to reduce leakage of thermal transferfluid. It should be understood by one of ordinary skill in the art thatfurther embodiments of the present invention exist where a proportioningvalve may have two independently controlled sliding plate valves whereone such valve is used to proportion flow between a straight-throughpath and a hot path, and another such valve is used to proportion flowbetween a straight-through path and a cold path. In addition, it shouldbe further understood by one of ordinary skill in the art that furtherembodiments of the present invention exist where three proportioningvalves may be used wherein one of such valves is used to regulate flowin a hot path, one to regulate flow in a cold path, and one to regulateflow in a straight-through path. In addition, in light of the above, oneof ordinary skill in the art will readily appreciate that furtherembodiments of the present invention exist utilizing any one of a numberof methods and apparatus to proportion fluid flow among parallel paths.

As shown in FIG. 1, straight-through path 1070 is shown as a channelfrom proportioning valve 1060 to junction 1240 of hot path 1080, coldpath 1090, and straight-through path 1070. A path from junction 1240 tothermal head 1020 may be referred to as a combined path for flow ofthermal transfer fluid from hot path 1080, cold path 1090, andstraight-through path 1070. However, further embodiments of the presentinvention exist wherein straight-through path 1070 is modified toaccelerate controlled changes in temperature of thermal head 1020 and toincrease its heating and cooling capacity. In accordance with one ormore such embodiments, a counter-flow heat exchanger may be disposed instraight-through path 1070 (in place of, or in addition to, counter-flowheat exchanger 530) to recover heat from the thermal transfer fluidflowing out from thermal head 1020 (i.e., thermal transfer fluid flowinginto channel 1055 shown in FIG. 1). In accordance with one suchembodiment, thermal transfer fluid exhausted from thermal head 1020would pass through a primary side of the counter-flow heat exchanger andthermal transfer fluid flowing between proportioning valve 1060 andjunction 1240 would pass through a secondary side of the counter-flowheat exchanger. In accordance with one or more such embodiments, thecounter-flow heat exchanger in straight-through path 1070 wouldaccomplish a function similar to that accomplished by counter-flow heatexchanger 530, particularly near equilibrium where no thermal transferfluid flows through hot path 1080 or cold path 1090. In addition, inlight of the above, one of ordinary skill in the art will readilyappreciate that further embodiments of the present invention existutilizing any one of a number of methods and apparatus for thermalconditioning to modify temperature of thermal transfer fluid in thestraight-through path.

As shown in FIG. 1, and in accordance with one or more embodiments ofthe present invention, thermal transfer fluid from each of hot path1080, cold path 1090, and straight-through path 1070 are mixed atjunction 1240. As one of ordinary skill in the art can readilyappreciate, check valves (not shown) or flow control valves may bedisposed in one or more paths, for example and without limitation,before junction 1240 to prevent reverse flow of the thermal transferfluid. In addition, and in accordance with one or more furtherembodiments of the present invention, additional heat may be added to,or removed from, the thermal transfer fluid after confluence at junction1240 to trim the temperature of the thermal transfer fluid flowing tothermal head 1020 using any one of a number of mechanisms. For exampleand without limitation, a resistive heater may be disposed in thermalcontact with the combined path leading to thermal head 1020 to provide amechanism to add a controlled amount of heat to the thermal transferfluid, wherein electrical current applied to the resistive heater isregulated by controller 1100 according to any one of a number offeedback algorithms that are well known to those of ordinary skill inthe art of temperature control. In further addition, and in accordancewith one or more further embodiments of the present invention,temperature TT4(t) of thermal head 1020 is regulated by routing thethermal transfer fluid so that it flows from junction 1240, though asecondary side of a counter-flow heat exchanger (not shown), and then tothermal head 1020. In accordance with one or more such embodiments,thermal transfer fluid exhausted (i.e., output) from thermal head 1020is routed through a primary side of the counter-flow heat exchanger, andfrom there to pump assembly 525. In accordance with one or more furthersuch embodiments, an array of thermoelectric coolers (“TECs”) may bedisposed between a primary side and a secondary side of the counter-flowheat exchanger so that one set of junctions of the TECs is thermallyconnected to the primary side of the counter-flow heat exchanger, andthe opposite set of junctions of the TECs is thermally connected to thesecondary side of the counter-flow heat exchanger. Then, controller 1100would cause an electrical current of a controlled level to be applied tothe TECs to cause them to extract heat from the thermal transfer fluidflowing in secondary side of the counter-flow heat exchanger, and to addheat to the thermal transfer fluid flowing in the primary side of thecounter-flow heat exchanger, thereby cooling the thermal transfer fluidflowing to thermal head 1020 by a controlled amount. The operation ofTECs disposed between the primary and the secondary sides ofcounter-flow heat exchangers is disclosed in U.S. Pat. No. 4,065,936,which patent is incorporated herein by reference.

Thus, as one of ordinary skill in the art can readily appreciate fromthe description above, controller 1100 controls the operation ofapparatus 1000 to set TT4 to one or more predetermined temperatures andto maintain TT4 at the predetermined temperatures for predeterminedamounts of time, which operation may be set by logic or software thatcontrols the operation of controller 1100 in the manner described above.In addition, and in accordance with one or more embodiments of thepresent invention, controller 1100 may control a sequence of temperaturechanges in accordance with a “recipe” or “script” or “profile” that maybe input using an operator interface terminal in accordance with any oneof a number of methods that are well known to those of ordinary skill inthe art. For example, and without limitation, in accordance with one ormore embodiments of the present invention, controller 1100 may executesoftware that interfaces to an operator via an operator interfaceterminal. The software may include a commercial operating system suchas, for example and without limitation, an appropriate version of aWindows™ operating system and custom software developed routinely andwithout undue experimentation by one of ordinary skill in the art toperform functions of controller 1100. In accordance with one or moresuch embodiments, a touch screen may be used to simplify operation, buta keyboard/mouse interface may be used as well. As will be readilyappreciated by one of ordinary skill in the art, a variety of othersoftware environments and user interfaces could also be used. Inaccordance with one or more such embodiments, the software may enable“profiles” to be defined and stored, which profiles specifytemperatures, how long to maintain the temperatures, and how to changeto new temperatures. Typically, this can be time related, or advanced bysignals from an external source, such as automatic test equipment usedto test DUT 1010. Thus, in accordance with one or more such embodiments,controller 1100 determines when and how long to maintain the temperatureof thermal head 1020, and causes that to occur. In accordance with oneor more further embodiments of the present invention, controller 1100operates in response to one or more testing criteria, operatingconditions, or feedback signals. For example, in addition to thedescription above, controller 1100 may operate in response to any of thefollowing: a test temperature setting associated with a current testingspecification for DUT 1010; an input signal utilized by DUT 1010, forexample and without limitation, an input power signal, an input voltage,or an input current; a signal indicative of a real-time operatingtemperature of DUT 1010; a signal indicative of a real-time operatingtemperature of an internal component of DUT 1010, for example andwithout limitation, a semiconductor die; an RF signature of the DUT; orthe like. In accordance with one or more embodiments of the presentinvention, controller 1100 communicates with a test control system. Inaccordance with one or more such embodiments, the test control systemwould carry out appropriate tests on DUT 1010 while apparatus 1000 wouldcontrol the temperature of thermal head 1020. As such, these two controlsystems might communicate or otherwise coordinate their activities. Inaccordance with one or more further embodiments of the presentinvention, the two systems are separate and have no directcommunication, and in accordance with one or more still furtherembodiments of the present invention, the two systems are fullyintegrated.

As one of ordinary skill in the art will readily appreciate, thechannels and/or conduits for flow of thermal transfer fluid between thecomponents of apparatus 1000 described may be, for example and withoutlimitation, smooth-bore PTFE hose with stainless steel wire braidreinforcing (available from McMaster-Carr Corporation of Los Angeles,Calif.). In addition, as was described above, temperatures TT1 and TT2of hot path 1080 and cold path 1090, respectively, refer to temperaturesof at least a portion of the paths, for example and without limitation,a portion of the paths between the valve assembly and junction 1240 thatpass through the heat exchangers.

FIG. 3 is a schematic representation of apparatus 2000 that isfabricated in accordance with one or more further embodiments of thepresent invention, which apparatus 2000 is useful to control temperature(for example and without limitation, set and maintain temperature) ofDUT 1010. Apparatus 2000 shown in FIG. 3 is the same as apparatus 1000shown in FIG. 1 except that pump unit 2500 of apparatus 3000 is used inplace of pump assembly 525 of apparatus 1000, and pulse-width modulatedvalves 2100, 2110, and 2120 are used in place of proportioning valve1060 of apparatus 1000—as such, the same numbers are utilized for thesame components in FIGS. 1 and 3. It should be noted that pulse-widthmodulated valves 2100, 2110, and 2120 may also be referred to herein asa valve assembly that operates in response to controller 1100. Thus, inaccordance with one or more embodiments of the present invention, and asshown in FIG. 3, pulse-width modulated valves 2100, 2110, and 2120 areused to control flow of thermal transfer fluid into straight-throughpath 1070, hot path 1080, and cold path 1090, respectively. As furthershown in FIG. 3, thermal transfer fluid exhausted from thermal head 1020of a test head is pumped back to valves 2100, 2110, and 2120, whichvalves are connected in parallel.

Pulse width modulated valves 2100, 2110, and 2120 are shown in FIG. 3 ina configuration wherein thermal transfer fluid is circulated from valve2100 to straight-through path 1070, through junction 1240, throughthermal head 1020 of a test head, through pump unit 2500, and back tovalve 2000. In an ideal case, no heat is gained or lost by the thermaltransfer fluid as it circulates. In apparatus 2000 shown in FIG. 3, pump2510 is exposed to thermal transfer fluid at the temperature at which itis exhausted from thermal head 1020 of the test head, thereby heating orcooling pump unit 2500 to the temperature of the circulating thermaltransfer fluid. A counter-flow heat exchanger may be necessary in thefluid circuit to and/or from pump 2510 in embodiments for which a rangeof temperature from that of hot path 1080 to that of cold path 1090 isrelatively large. Thus, heat capacity and temperature limits of pumpunit 2500 determine the need to use a counter-flow heat exchanger inpump unit 2500 in the manner that counter-flow heat exchanger 530 wasused in pump assembly 525 of apparatus 1000 shown in FIG. 1. Forexample, and without limitation, if the temperature of thermal transferfluid flowing from thermal head 1020 exceeds either the high temperaturelimit or the low temperature limit of pump 2510, then one would need touse a counter-flow heat exchanger. In addition, it should be readilyappreciated by those of ordinary skill in the art that the variousalternatives discussed above with respect to apparatus 1000 shown inFIG. 1 are also applicable to apparatus 2000 shown in FIG. 3.

In accordance with one or more embodiments of the present invention, thethermal transfer fluid is liquid 3M™ FLUORINERT™ Electronic LiquidFC-77, a fluorocarbon based fluid. In addition, in accordance with oneor more such embodiments, pump 2510 is a gear pump available fromMicropump, Inc. of Vancouver, Wash., and suitable pulse-width modulatingvalves are available from Parker Hannifin Corporation of Cleveland,Ohio. The flow of the FLUORINERT™ fluid to each of hot path 1080,straight-path 1070, and cold path 1090 is regulated by controller 1100causing activation of each of valves 2110, 2100, and 2120, respectively.To do this, controller 1100 causes appropriate pulse width modulatedexcitation signals to be applied to the valves. For example, and withoutlimitation, an electrical excitation having a short pulse width causes avalve to open momentarily, allowing a controlled amount of FLUORINERT™thermal transfer fluid to flow into a selected path. Advantageously, apulse width modulated valve is able to open and close at about 10 timesper second, allowing accurate control of the average flow rate throughthe valve. Thus, in accordance with one or more such embodiments of thepresent invention, controller 1100 causes pulses to be supplied to oneor more of the valves to regulate temperature TT3 of the thermaltransfer fluid that flows to thermal head 1020 of the test head, therebycontrolling temperature TT4 of thermal head 1020.

As was described above, in fabricating one or more embodiments of thepresent invention, one may utilize a heat transfer apparatus that is athermal head (described above) wherein a thermal transfer fluid flowsthrough the thermal head and, to effectuate heat transfer, a device isbrought into thermal contact with the thermal head. It should beunderstood by one of ordinary skill in the art that the term thermalcontact refers to contact between a first and a second constituentwhereby heat may be transferred therebetween primarily by conduction,even indirectly (i.e., by conduction through intermediary materials).However, in fabricating one or more further embodiments of the presentinvention, one may utilize a heat transfer apparatus that comprises astructure which includes a chamber (for example and without limitation,a test chamber), which chamber is suitable to contain the device or aportion of the device, and which structure may further includeconduit(s) to and/or from the chamber, wherein thermal transfer fluidflows through the chamber (and conduits). In accordance with one or moresuch embodiments, the thermal transfer fluid may come into thermalcontact (for example and without limitation, direct contact) with thedevice or a portion of the device. Further, in accordance with one ormore such embodiments, the chamber may be a vacuum tight sealed chamber.Still further, in accordance with one or more such embodiments, thethermal transfer fluid may be, in addition to any other thermal transferfluid described herein, for example, and without limitation, helium,helium mixtures, nitrogen, carbon dioxide, hydrogen, mixtures of theforegoing, or an inert gas or gases other than helium.

Further, although embodiments of the present invention were describedwherein a thermal head was utilized, it should be understood by one ofordinary skill in the art that the term thermal head may include thermalheads whose purpose is not only to regulate temperature but to includeapparatus for carrying out any one of a number of tests that are wellknown to those of ordinary skill in the art.

Embodiments have been described above for controlling temperature of adevice under test (“DUT”) wherein the device under test is functionallytested by automatic test equipment, and wherein the DUT is, for exampleand without limitation, a packaged integrated circuit (“IC”) device or adie of a semiconductor wafer. However, it should be understood by thoseof ordinary skill in the art that method and apparatus fabricated inaccordance with one or more embodiments of the present invention mayapply to a variety of different fields, applications, industries, andtechnologies. As such, one or more embodiments of the present inventioncan be used with any system in which temperature is either set and/orcontrolled. This includes many different processes and applicationsinvolved in semiconductor fabrication, testing, and operation.

Embodiments of the present invention described above are exemplary. Assuch, many changes and modifications may be made to the disclosure setforth above while remaining within the scope of the invention. Inaddition, materials, methods, and mechanisms suitable for fabricatingembodiments of the present invention have been described above byproviding specific, non-limiting examples and/or by relying on theknowledge of one of ordinary skill in the art. Materials, methods, andmechanisms suitable for fabricating various embodiments or portions ofvarious embodiments of the present invention described above have notbeen repeated, for sake of brevity, wherever it should be wellunderstood by those of ordinary skill in the art that the variousembodiments or portions of the various embodiments could be fabricatedutilizing the same or similar previously described materials, methods ormechanisms. Further, as is apparent to one skilled in the art, theembodiments may be used for making connections to semiconductor devices,electronic devices, electronic subsystems, cables, and circuit boardsand assemblies.

As one or ordinary skill in the art will readily appreciate, one or moreembodiments of the present invention may include any number of fluidseals, gaskets, adhesives, washers, or other elements that function toseal the assembly and to prevent thermal transfer fluid from leaking(internally or externally).

The scope of the invention should be determined with reference to theappended claims along with their full scope of equivalents.

What is claimed is:
 1. An apparatus to control device temperaturecomprises: a thermal fluid path which transfers a thermal transfer fluidso that the device is heated or cooled in response to the thermaltransfer fluid; a temperature sensor in communication with a controllerand being operable to sense a temperature in response to the thermalfluid path; a first path in thermal contact with a heating assembly,which first path is fluidly connected at a first end to the thermalfluid path; a second path in thermal contact with a cooling assembly,which second path is fluidly connected at a first end to the thermalhead fluid path; a third path fluidly connected at a first end to thethermal fluid path; a supply apparatus which supplies thermal transferfluid to a confluence of a second end of the first path, a second end ofthe second path, and a second end of the third path; and a valveassembly being operable, in response to the controller, to regulate flowof thermal transfer fluid flow through the first, the second, and thethird paths.
 2. The apparatus of claim 1 which comprises a thermaltransfer fluid and wherein the thermal transfer fluid is a gas or aliquid.
 3. The apparatus of claim 1 wherein the supply apparatusincludes a fluid pump.
 4. The apparatus of claim 1 wherein the supplyapparatus comprises: a counter-flow heat exchanger with a primarycircuit disposed to receive a thermal transfer fluid from the thermalfluid path; and a secondary circuit disposed to input thermal transferfluid from a pump and output thermal transfer fluid to the confluence ofthe first, the second and the third paths.
 5. The apparatus of claim 1wherein the valve assembly includes: a first valve operable in responseto the controller wherein the first valve regulates flow of thermaltransfer fluid through the first path; a second valve operable inresponse to the controller wherein the second valve regulates flow ofthermal transfer fluid through the second path; and a third valveoperable in response to the controller wherein the third valve regulatesflow of thermal transfer fluid through the third path
 6. The apparatusof claim 1 further comprising: a first path temperature sensor incommunication with the controller and being operable to sense atemperature in response to the first path; and a second path temperaturesensor in communication with the controller and being operable to sensea temperature in response to the second path; wherein: the heatingassembly is operable in response to the controller; and the coolingassembly is operable in response to the controller.
 7. The apparatus ofclaim 5 wherein one or more of the first, second and third valves is apulse-width modulated valve.
 8. The apparatus of claim 2 wherein thethermal transfer fluid comprises nitrogen.
 9. The apparatus of claim 1wherein the thermal fluid path is adapted to bring the thermal fluidinto direct contact with the device.
 10. The apparatus of claim 1wherein the thermal fluid path flows into a thermal head having conduitsor fluid channels therein.